Intermediate Jacobians and Abel-Jacobi Maps

Intermediate Jacobians and Abel-Jacobi Maps Patrick Walls April 28, 2012

Introduction Let X be a smooth projective complex variety.

Introduction Let X be a smooth projective complex variety. Intermediate Jacobians are complex tori defined in terms of the Hodge structure of X. Abel-Jacobi Maps are maps from the groups of cycles of X to its Intermediate Jacobians.

Hodge Decomposition Let X be a smooth projective complex variety of dimension n. The Hodge decomposition is a direct sum decomposition of the complex cohomology groups H k (X, C) = H p,q (X ), 0 k 2n. p+q=k The subspaces H p,q (X ) consist of classes [α] of differential forms that are representable by a closed form α of type (p, q) meaning that locally α = f I,J dz I dz J I, J {1,..., n} I = p, J = q for some choice of local holomorphic coordinates z 1,..., z n.

Hodge Decomposition (Continued) The subspaces H p,q (X ) satisfy the Hodge symmetry H p,q (X ) = H q,p (X ) where α α is the natural action of complex conjugation on H k (X, C) = H k (X, R) R C.

Hodge Decomposition (Continued) The subspaces H p,q (X ) satisfy the Hodge symmetry H p,q (X ) = H q,p (X ) where α α is the natural action of complex conjugation on H k (X, C) = H k (X, R) R C. Since real forms are conjugate invariant, we may write [α] H 2k (X, R) in terms of the Hodge decomposition α = α 2k,0 + + α k+1,k 1 + α k,k + α k+1,k 1 + + α 2k,0 and for [β] H 2k 1 (X, R) in odd degree β = β 2k+1,0 + β 2k,1 + + β k+1,k + β k+1,k + + β 2k,1 + β 2k+1,0.

Hodge Decomposition (Continued) The Hodge decomposition defines a filtration 0 F k H k (X ) F k 1 H k (X ) F 0 H k (X ) = H k (X, C) where F r H k (X ) = p r H p,k p (X ), 0 r k.

Hodge Decomposition (Continued) The Hodge decomposition defines a filtration 0 F k H k (X ) F k 1 H k (X ) F 0 H k (X ) = H k (X, C) where F r H k (X ) = p r H p,k p (X ), 0 r k. The cohomology groups of odd degree satisfy H 2k 1 (X, C) = F k H 2k 1 (X ) F k H 2k 1 (X ) and the natural map H 2k 1 (X, R) H 2k 1 (X, C)/F k H 2k 1 (X ) is an isomorphism of real vector spaces.

Intermediate Jacobians The rank of H 2k 1 (X, Z) is the dimension of H 2k 1 (X, R) therefore H 2k 1 (X, Z) defines a full lattice L k = im ( H 2k 1 (X, Z) H 2k 1 (X, C)/F k H 2k 1 (X ) ) in the complex vector space V k = H 2k 1 (X, C)/F k H 2k 1 (X ). Definition The kth intermediate Jacobian of X is the complex torus J k (X ) = V k /L k = H 2k 1 (X, C)/(F k H 2k 1 (X ) H 2k 1 (X, Z)).

Intermediate Jacobians (Continued) Poincaré duality asserts H 2n 2k+1 (X, C) = H 2k 1 (X, C) and so F n k+1 H 2n 2k+1 (X ) = H 2k 1 (X, C)/F n k+1 H 2n 2k+1 (X ).

Intermediate Jacobians (Continued) Poincaré duality asserts and so H 2n 2k+1 (X, C) = H 2k 1 (X, C) F n k+1 H 2n 2k+1 (X ) = H 2k 1 (X, C)/F n k+1 H 2n 2k+1 (X ). We see that F n k+1 H 2n 2k+1 (X ) = F k H 2k 1 (X ) since a form of type (p, 2n 2k + 1 p) with p n k + 1 wedged with a form of type (p, 2k 1 p ) with p k is a form of type (p, 2n p ) with p n + 1 which must be zero since X has complex dimension n.

Intermediate Jacobians (Continued) Poincaré duality further states that H 2k 1 (X, Z) = H 2n 2k+1 (X, Z) therefore we have an equivalent form of J k (X ).

Intermediate Jacobians (Continued) Poincaré duality further states that H 2k 1 (X, Z) = H 2n 2k+1 (X, Z) therefore we have an equivalent form of J k (X ). Definition The kth intermediate Jacobian of X is the complex torus J k (X ) = F n k+1 H 2n 2k+1 (X ) /H 2n 2k+1 (X, Z). We abuse notation here as Voisin does when we do not distinguish between H 2k 1 (X, Z) and its image in H 2k 1 (X, C) and similarly for H 2n 2k 1 (X, Z) and its image in H 2n 2k 1 (X, C).

Cycle Classes We can attach an integral cohomology class [Z] H 2k (X, Z) to a cycle Z of codimension k in X and extending by Z-linearity defines a homomorphism called the cycle class map. Z k (X ) H 2k (X, Z) : Z [Z] The image of [Z] in H 2k (X, R) is described by Poincaré duality H 2k (X, R) = H 2n 2k (X, R). The class [Z] is the form of degree 2k such that [Z], α X = α, for all α H 2n 2k (X, R), Z sm where Z sm is the smooth locus of Z.

Constructing the Abel-Jacobi Map The kernel of the cycle class map is the group of null-homologous cycles Z k (X ) hom = ker ( Z k (X ) H 2k (X, Z) ). In other words, if Z Z k (X ) hom there exists some chain C Z of dimension 2n 2k + 1 such that C Z = Z. Integrating over C Z defines a functional on forms of degree 2n 2k + 1 ( ) ω A 2n 2k+1 (X ) C Z ω therefore we would like to attach to Z a cohomology class in degree 2k 1 modulo the ambiguity in choosing the chain C Z.

Constructing the Abel-Jacobi Map (Continued) Integrating over C Z will only define a functional on cohomology if it is zero on exact forms. If ω = dψ for some form ψ of degree 2n 2k, we have ω = ψ C Z Z which vanishes if ψ is of type (p, q) (n k, n k) since then ψ Z = 0 because Z has complex dimension n k. Therefore, integrating over C Z does not define a functional on the whole space H 2n 2k+1 (X, C) but we can choose a piece of the cohomology on which it does.

Constructing the Abel-Jacobi Map (Continued) It can be shown that F n k+1 H 2n 2k+1 (X, C) = F n k+1 A 2n 2k+1 (X ) ker d d F n k+1 A 2n 2k (X ) and we have shown that C dψ = 0 for all ψ F n k+1 A 2n 2k (X ) Z therefore ( ) ω F n k+1 H 2n 2k+1 (X, C). C Z ω

Constructing the Abel-Jacobi Map (Continued) If C Z is another chain such that C Z = Z then (C Z C Z ) = 0 therefore C Z C Z defines a class in H 2n 2k+1(X, Z) and so ( ) ω F n k+1 H 2n 2k+1 (X ) /H 2n 2k+1 (X, Z) C Z ω and we recognize this quotient as the kth intermediate Jacobian of X.

Constructing the Abel-Jacobi Map (Continued) Let d k = dim C F n k+1 H 2n 2k+1 (X ) and choose a basis ω 1,..., ω dk F n k+1 H 2n 2k+1 (X ).

Constructing the Abel-Jacobi Map (Continued) Let d k = dim C F n k+1 H 2n 2k+1 (X ) and choose a basis ω 1,..., ω dk F n k+1 H 2n 2k+1 (X ). Choose a basis of the homology group let Ω i = γ 1,..., γ 2dk H 2n 2k+1 (X, Z), ( ω γi 1,..., ) ω γi dk C d k and let Λ = span Z {Ω 1,..., Ω 2dk } C d k.

Example: The Albanese Variety (k = n) In the case k = n, a cycle Z Z n (X ) is a point and the complex torus J n (X ) = Alb X is called the Albanese variety of X.

Example: The Albanese Variety (k = n) In the case k = n, a cycle Z Z n (X ) is a point and the complex torus J n (X ) = Alb X is called the Albanese variety of X. We may fix a point x 0 X and consider the null-homologous 0-cycles (x) (x 0 ) as x varies in X. The Abel-Jacobi Map on these cycles then defines a holomorphic map alb X : X Alb X : x Φ n X((x) (x 0 )) which satisfies the following universal property. Theorem If f : X T is a holomorphic map from X to a complex torus T such that f (x 0 ) = 0, then there exists a unique morphism of complex tori g : Alb X T such that g alb X = f.

The Simplest Case (k=n=1) The chain C (x) (x0 ) varying in a curve X of genus 1 as x varies in X :

Holomorphic Families of Cycles The Albanese map is the simplest example of the Abel-Jacobi map applied to a holomorphic family of cycles. More generally, we can consider a holomorphic family (Z y ) y Y of cycles of codimension k in X parametrized by a connected complex manifold Y. Theorem (Griffiths, 1968) Let X be a complex Kähler manifold, let Y be a connected complex manifold and let Z Y X be a cycle of codimension k and flat over Y. The fibers Z y are all homologous in X and, given a reference point y 0 Y, the map is holomorphic. φ : Y J k (X ) : y Φ k X(Z y Z y0 )

Rational Equivalence Theorem If Z Z k (X ) is rationally equivalent to 0 then Φ k X (Z) = 0. In particular, the Abel-Jacobi map factors through the (null-homologous) Chow group Φ k X : CH k (X ) hom J k (X ). The idea is that if Z 1 and Z 2 are rationally equivalent then these cycles lie in a family of cycles parametrized by P 1 and the Abel-Jacobi map then defines a holomorphic map Φ k X : P1 J k (X ). But such a holomorphic map must be constant so Φ k X (Z 1) = Φ k X (Z 2). We now have a tool with which to study the Chow groups CH k (X ) hom in terms of the simpler objects J k (X ).

Applications: The Work of Schoen The recurring fantasy of Bloch conjectures the relationship rank CH k (X F ) hom = ord s=k L(H 2k 1 (X F ), s) for a smooth projective variety X defined over a number field F. The rough idea of (a part of) Schoen s work is to consider a specific moduli space W of elliptic curves and to construct a large class of cycles over an imaginary quadratic field F and of codimension 2 in W using of the theory of complex multiplication. Since proving rational equivalence amongst these cycles is much too difficult, the idea is to provide modest numerical evidence that the rank of the subgroup these cycles generate, via the Abel-Jacobi map, in the intermediate Jacobian J 2 ( W ) is 1 when the corresponding L-function is known to have a simple zero at s=2.

References VOISIN, C., Hodge Theory and Complex Algebraic Geometry I, Cambridge studies in advanced mathematics vol.76, Cambridge University Press, 2002. SCHOEN, C., Complex Multiplication Cycles and a Conjecture of Beilinson and Bloch, Transactions of the American Mathematical Society, Vol.339, No.1, 1993.